In many applications there is a need for distributed Private Branch Exchanges (PBXs) in corporate sites which are remotely located from one another. Ideally, a caller at one such facility should be able to directly dial an extension at another facility. This, of course, can be accomplished by placing a call over the public telephone network after first gaining access to an outgoing telephone central office trunk and then dialing the appropriate area code and office code plus the extension desired. In addition, if the facilities are interconnected by a private telephone network, then the call can also be placed by dialing a single digit which gains access to a trunk connected to the private network and then dialing the location code which is peculiar to the private network plus the desired extension. For example, to place a call to a particular business telephone set over the public network, a person at a PBX dials 9, then the digit 1, area code 303, central office number 538, and the extension. Whereas to place such a call over the private telephone network which services that business telephone, a person dials 8, location code 374 and the extension.
In PBX systems, it is known to do automatic route selection among the various trunks which make up the public telephone system in order to minimize the cost of a long-distance telephone call. For example, in the United States, American Telephone and Telegraph, Inc. provides trunks of different classes which are leased at different rates, depending on the class of service. Specifically, Full Business Day (FBD), Wide-Area Telephone System (WATS) trunks are available at one tariff schedule, Measured Time (MT) WATS trunks are available at a different schedule, and Direct-Distance Dialing (DDD) trunks are available at still another tariff schedule. One such prior art, automatic route selection method, is disclosed in U.S. Pat. No. 4,163,124, which issued to C. H. Jolissaint. The automatic route selection described in the Jolissaint patent is utilized in another prior art PBX arrangement as disclosed in U.S. Pat. No. 4,313,036, which issued to M. D. Jabara, C. H. Jolissaint, D. Lieberman, and J. D. Edwards. The Jabara system utilizes Jolissaint's route selection method for selecting routes among distributed PBX systems.
In the corporate setting, the requirement of establishing a telephone connection between two corporate sites by dialing either the public network or private network set of numbers is troublesome, because two sets of numbers must be remembered or looked up for each site before a call can be placed. Whereas this problem can be handled where there are only a limited number of PBX sites, it becomes unacceptable when there are hundreds of PBX sites. In particular, in the retail corporate environment where the public telephone number is listed in numerous documents, the trouble of having to look up the private network number is unacceptable. The result is that employees use the public telephone network, resulting in higher costs to the retail corporation since it is more expensive to make calls over the public telephone network rather than the private telephone network. Jabara's system allows for automatic route selection with respect to the public network with a given public network number but does not make provisions for converting a public network number into a private network number in order to perform route selection over a private network.
Another problem which has existed in prior art PBX systems is the problem of restricting outgoing calls with sufficient specificity to allow employee access to the public telephone network but to prohibit certain popular but not work-related telephone numbers such as the weather or sports information telephone numbers. Within the prior art, it is known to be able to restrict all outgoing calls or to be able to restrict calls to given central offices. However, it is not known to be able to restrict calls to a particular telephone number or to a particular set of telephone numbers without restricting the calls to that particular central office. These limitations present serious problems for the employer who wishes to restrict employee access to a particular number not work-related in a large metropolitan area in which his employees may well have business reasons for calling numbers within the same central office as that particular telephone number.
Therefore, there exists a need for a PBX system which is capable of performing automatic route selection not only within different classes of the trunks which are provided by the public telephone network but also having the capability to provide a communication path over a private network in response to a dialed public telephone number. Furthermore, there exists a need for the capability of restricting outgoing calls from a PBX system on the basis of dialed telephone numbers. Further, these capabilities must be provided in such manner so as to minimize the amount of memory required and the amount of computer time necessary to execute the required software procedures.